Photoshop CurvesColor Correction

The Curves control can also be used to neutralize a
color cast. Two methods are discussed here. The first method
uses the eyedroppers on the Curves dialog box. The second uses the
individual color channels on the Curves dialog box. I find either one
easier to use if I first have created light, dark and mid tone targets in the
image. Targeting these areas is discussed on the
Finding Lightest and Darkest Areas page.

Neutralizing A Color Cast

Figure 1. Color Image With a Blue/Cyan Cast

Figure 2. Curves Dialog Box

This example uses the same image used in the Levels color correction section.
As can be seen in Figure 1, this image has a blue/cyan cast to it.

Eyedropper Technique

In Figure 2 can be seen the Curves dialog box. In the dialog box are three
eyedroppers. The purpose of the black eyedropper
is to take whatever part of the image it is clicked on and remap it to black.
The purpose of the white eyedropper
is to take whatever part of the image it is clicked on and remap it to white.
The gray eyedropper
behaves differently. It does not remap pixel tones to mid tone.
Instead, it takes whatever part of the image it is clicked on and neutralizes
its color while maintaining the original tone. Neutralizing a color means
making it gray, but not a mid tone gray. I have noticed that, except for
solid colors, the gray eyedropper does not absolutely neutralize a color and
there is usually a slight shift in tone.

Using the techniques to identify the lightest and darkest areas of an image
(described on the
Finding Lightest and Darkest Areas page), I created the three Color Sampler
targets
shown in Figure 1. Target #1 is a white area. Target #2 is a dark
area. Target #3 is a medium toned area whose color we want to be neutral.
We will be using all three eyedroppers to neutralize the color cast.

Here are the steps I used to neutralize the color cast in Figure 1 using the
Curves eyedroppers.

Make sure the caps lock key is on. This will change the eyedropper
cursor to a crosshair
.

Create a new adjustment layer. Layer > New Adjustment Layer > Curves.

In the dialog box, click the white eyedropper
.
It is the eyedropper on the right in Figure 2.

Place the eyedropper cursor over the image. When we do this, the cursor
will become a crosshair. This is the same crosshair used by the Color
Sampler targets.

Position the eyedropper crosshair
over the white target, in this case #1. When the two crosshairs are
perfectly aligned, they will merge and become one.

Click once using the left mouse button. Do not hold down the
Shift key while clicking.

Repeat for the black eyedropper
using the black target, in this case #2.

Repeat for the gray eyedropper
using the mid tone target, in this case #3.

The result can be seen in Figure 3. Figure 3 has had no other adjustments
other than the Curves eyedroppers. Mouse over Figure 3 to see
corrected/uncorrected versions.

Like the example in Levels, the pine tree in the foreground has a nice neutral
color. The reds, yellows, whites and greens are also truer.

Color Channels Technique

The Color Channels technique is more involved but is used when we need
precise control. Even though Color Sampler targets
were not mandatory when using the eyedropper technique, they are necessary when
using the Color Channel technique.

I find the Color Channel technique with Curves to be slightly more complex than
Levels. However, it has an advantage over Levels. If we use a mid
tone Color Sampler target to help neutralize a color, with Levels we need to get
the target as close to mid tone as possible since Levels can only adjust black,
white and mid tone. Since Curves allows us to work with any point on the
curve, we do not have to be as precise. However, for the best results, I
still look for a black and white point at a minimum and also add a mid tone
point.

Figure 4. Info Panel of Uncorrected Image

Once the Color Sampler targets have been created, we will use the
information they provide in the Adobe Photoshop Info panel to reduce the color
cast. We can have up to four color sampler targets in a Photoshop
document. The image in Figure 1 has three. They are labeled #1, #2
and #3 in Figure 4. When the Curves dialog box is open, the Info panel
shows before and after tonal values for each of the targets. The before
values are on the left and the after values are listed on the right.

In Figure 4, color sampler #1 has a red value of 201, a green
value of 255 and a blue value of 254. Target #1 has a cyan
cast to it because green and blue are the dominant color channels
and they are of similar amount (green + blue = cyan). Color
sampler #2 in Figure 4 has red and green values of 0 and a blue
value of 15. Thus, target #2 has a blue tint to it.
Target #3, which is the bark of the pine tree, also has a cyan tint
to it because of the dominance of the green and blue channels.

When neutralizing a color cast, the intent is to change the RGB
after values to be identical to each other. For the white
point (target #1), we want to make each of the RGB after values
equal to the highest RGB before value, which is 255 in this
case. For the black point, we want to make each of the RGB
after values equal to the lowest RGB before value, in this
case 0. For target #3, we do not necessarily want to make the
three channels equal to each other, for this would make the tree
bark a true gray. When changing the image for target #3, we
want to adjust it enough to leave some color but not make it
monochrome.

To remove the color cast using the Channels technique, follow these
steps.

Using the Info panel, note the RGB values of the color sampler
targets. They are in the format before/after. The intent is to change the
RGB after values to be identical to each other for the black and white
targets. For the black point, we want to make each of the RGB after values
equal to the
lowest RGB before value. For the white point, we want to make each
of the RGB after values equal to the highest RGB before value.

To correct the black point. Using the Channel drop down box, select the
one or two channels, in turn, that do not have the lowest RGB before value.
In this case, it is the blue channel. Ctrl + Tab (Command + Tab) until the
black point anchor is highlighted. Click in the Input box. Using
the up arrow key, change the number until the after value in the Info
panel matches the lowest before value. In this example, we want to change
the blue channel until its after value in the Info panel equals 0.
Referring to Figure 5, we note that the red channel black point was not
changed. The green channel black point was not changed either. The blue
channel black point was changed to 15.

Red channel, Black pointInput=0, Output=0

Green channel, Black pointInput=0, Output=0

Blue channel, Black pointInput=15, Output=0

Figure 5. Black point adjustments

To correct the white point. Using the same Curves adjustment
layer, select the one or two channels, in turn, that do not have the highest
RGB before value. In this case, it is the red and blue channels. Use Ctrl
+ Tab (Command + Tab) to highlight the white anchor. Click in the
Input box of the Curves dialog box. Using the down arrow key, change
the number until the after value in the Info panel matches the highest
before value. In this example, we want to change the value until the red
channel 201 in the Info panel equals 255. When we edit the blue channel
we want to change the blue channel white point value until the blue channel
254 equals 255. Looking at the three dialog boxes in Figure 6, we note that
the red channel white point anchor was changed to 201. The green channel
white point anchor was not changed. The blue channel white point was
changed to 254.

Red channel, White pointInput=201, Output=255

Green channel, White pointInput=255, Output=255

Blue channel, White pointInput=254, Output=255

Figure 6. White point adjustments

If further correction is still needed, then correct the mid point. This
change is more subjective than numerically precise. In this case we know
that the tree bark has a blue/cyan cast. To undo this dominance, we want
to change the values such that neither blue nor green + blue is dominant since
green + blue makes cyan.

To make the mid tone adjustment, we need to place anchor points on each
individual color channel's curve for the mid tone target. This is easily
accomplished by Ctrl + Shift + clicking (Command + Shift + clicking) the mid
tone target. In this case, target #3. With this one operation, an anchor
will be placed on the Red, Green and Blue channels. An anchor will not be
placed on the composite channel. This ensures the mid tone we are
correcting coincides with the target tones shown in the Info panel.

To make the adjustment, select each color channel in turn and click in
the
Output box this time. Using the up/down arrow keys, change the
Output number until the desired effect is achieved. Referring to Figure 7,
the red channel was changed to 180. The green channel was changed to 119.
The blue channel was changed to 114.

Red channel, Mid toneInput=127, Output=180

Green channel, Mid toneInput=127, Output=119

Blue channel, Mid toneInput=127, Output=144

Figure 7. Mid tone adjustments

The corrected image can be seen in Figure 8. Mouse over Figure 8 to see
both the corrected and the uncorrected versions of the image. Figure 9
shows the Info panel of the corrected image. Target #1 has been
neutralized to a true white. Target #2 has been neutralized to a true
black. Target #3 was not neutralized. But that is intentional
since our objective was to
remove the dominant blue/cyan color cast in this area, not make it gray.

Comparison

For comparison purposes, lets see how the eyedropper and channels techniques
compare. In Figure 10 are the three Info panels. Figure 10A is the
Info panel for the uncorrected image. Figure 10B is the Info panel for
the image corrected by the Curves Eyedropper technique. Figure 10C is the
Info panel of the image corrected by the Curves Channels technique.
(Figure 10C is the same as Figure 9.)

Evaluating Figure 10A, we can tell the color cast was predominantly in the mid
tones since targets #1 and #2 (white and black areas respectfully) do not show
much deviation from neutral. Figure 10B shows that the Eyedropper
technique did a decent job of neutralizing the blue/cyan cast in the mid tones
(target #3). It also did a good job of neutralizing the white and black
points. However, the Channels technique, which is represented by Figure
10C, demonstrates the precise control this technique allows. The whites
and blacks were absolutely neutralized and the mid tone area was changed as
desired.

Figure 10A. Uncorrected Image

Figure 10B. Eyedropper Technique

Figure 10C. Channels Technique

Summary

The Color Channels technique can seem overwhelming to use. Therefore, a
suggested workflow for removing a color cast is as follows.

First, locate white and black areas in the image that should be
corrected and use the white
and black
eyedroppers and click these areas. If the results are unsatisfactory,...

Then locate a color in the image that should be neutral and is as close
to mid tone as possible. Use only the gray
eyedropper and click this area. If the results are still not
satisfactory,...

Then use the Channels technique.

Tip

The key to using color correction is to be sure to
target the correct areas. Do not target deep shadows or
specular highlights.